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前文再續，書接上一回【NioEventLoop】。
在研究NioEventLoop執行過程的時候，檢測IO事件（包括新連接），處理IO事件，執行所有任務三個過程。其中檢測IO事件中通過持有的selector去輪詢事件，檢測出新連接。這裏複用同一段代碼。
Channel的設計
在開始分析前，先了解一下Channel的設計
頂層Channel接口定義了socket事件如讀、寫、連接、綁定等事件，並使用AbstractChannel作爲骨架實現了這些方法。查看器成員變量，發現大多數通用的組件，都被定義在這裏
第二層AbstractNioChannel定義了以NIO，即Selector的方式進行讀寫事件的監聽。其成員變量保存了selector相關的一些屬性。
第三層內容比較多，定義了服務端channel（左邊繼承了AbstractNioMessageChannel的NioServerSocketChannel）以及客戶端channel（右邊繼承了AbstractNioByteChannel的NioSocketChannel）。
如何接入新連接?
本文開始探索一下Netty是如何接入新連接？主要分爲四個部分
1.檢測新連接
2.創建NioSocketChannel
3.分配線程和註冊Selector
4.向Selector註冊讀事件
1.檢測新連接
Netty服務端在啓動的時候會綁定一個bossGroup，即NioEventLoop,在bind()綁定端口的時候註冊accept（新連接接入）事件。掃描到該事件後，便處理。因此入口從：NioEventLoop#processSelectedKeys()開始。
private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
//省略代碼
// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
// to a spin loop
//如果當前NioEventLoop是workGroup 則可能是OP_READ，bossGroup是OP_ACCEPT
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
//新連接接入以及讀事件處理入口
unsafe.read();
}
}關鍵的新連接接入以及讀事件處理入口unsafe.read();
a).這裏的unsafe是在Channel創建過程的時候，調用了父類AbstractChannel#AbstractChannel()的構造方法，和pipeline一起初始化的。
protected AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}服務端：
unsafe 爲NioServerSockeChannel的父類AbstractNioMessageChannel#newUnsafe()創建,可以看到對應的是AbstractNioMessageChannel的內部類NioMessageUnsafe;
客戶端：
unsafe爲NioSocketChannel的的父類AbstractNioUnsafe#newUnsafe()創建的話，它對應的是AbstractNioByteChannel的內部類NioByteUnsafe
b).unsafe.read()
NioMessageUnsafe.read()中主要的操作如下：
1.循環調用jdk底層的代碼創建channel，並用netty的NioSocketChannel包裝起來，代表新連接成功接入一個通道。
2.將所有獲取到的channel存儲到一個容器當中，檢測接入的連接數，默認是一次接16個連接
3.遍歷容器中的channel,依次調用方法fireChannelRead，4.fireChannelReadComplete，fireExceptionCaught來觸發對應的傳播事件。
private final class NioMessageUnsafe extends AbstractNioUnsafe {
//臨時存儲讀到的連接
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
//服務端接入速率處理器
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
//while循環調用doReadMessages()創建新連接對象
do {
//獲取jdk底層的channel,並加入readBuf容器
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
//把讀到的連接做一個累加totalMessages，默認最多累計讀取16個連接，結束循環
allocHandle.incMessagesRead(localRead);
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
//觸發readBuf容器內所有的傳播事件:ChannelRead 讀事件
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i));
}
//清空容器
readBuf.clear();
allocHandle.readComplete();
//觸發傳播事件：ChannelReadComplete，所有的讀事件完成
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
//觸發傳播事件：exceptionCaught，觸發異常
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}而這一段關鍵代碼邏輯中 int localRead = doReadMessages(readBuf);它創建jdk底層channel並且用NioSocketChannel包裝起來，將該channel添加到傳入的容器保存起來，同時返回一個計數。
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
//將jdk底層的channel封裝到netty的channel，並存儲到傳入的容器當中
//this爲服務端channel
buf.add(new NioSocketChannel(this, ch));
//成功和創建 客戶端接入的一條通道，並返回
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}2.創建NioSocketChannel
通過檢測IO事件輪詢新連接，當前成功檢測到連接接入事件之後，會調用NioServerSocketChannel#doReadMessages()方法，進行創建NioSocketChannel,即客戶端channel的過程。
下面就來了解一下NioSocketChannel的主要工作：
.查看原代碼做了兩件事，調用父類構造方法，實例化一個NioSocketChannelConfig。
public NioSocketChannel(Channel parent, SocketChannel socket) {
super(parent, socket);
//實例化一個NioSocketChannelConfig
config = new NioSocketChannelConfig(this, socket.socket());
}1）、查看NioSocketChannel父類構造方法,主要是保存客戶端註冊的讀事件、channel爲成員變量，以及設置阻塞模式爲非阻塞。
public NioSocketChannel(Channel parent, SocketChannel socket) {
super(parent, socket);
//實例化一個NioSocketChannelConfig
config = new NioSocketChannelConfig(this, socket.socket());
}
protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
//傳入感興趣的讀事件：客戶端channel的讀事件
super(parent, ch, SelectionKey.OP_READ);
}
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent);
//保存客戶端channel爲成員變量
this.ch = ch;
//保存感興趣的讀事件爲成員變量
this.readInterestOp = readInterestOp;
try {
//配置阻塞模式爲非阻塞
ch.configureBlocking(false);
} catch (IOException e) {
try {
ch.close();
} catch (IOException e2) {
if (logger.isWarnEnabled()) {
logger.warn(
"Failed to close a partially initialized socket.", e2);
}
}
throw new ChannelException("Failed to enter non-blocking mode.", e);
}
}最後調用父類的構造方法，是設置該客戶端channel對應的服務端channel，以及channel的id和兩大組件unsafe和pipeline
protected AbstractChannel(Channel parent) {
//parent爲創建次客戶端channel的服務端channel（服務端啓動過程中通過反射創建的）
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}2）、再看NioSocketChannelConfig實例化。主要是保存了javaSocket，並且通過setTcpNoDelay(true);禁止了tcp的Nagle算法，目的是爲了儘量讓小的數據包整合成大的發送出去,降低延時.
private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
super(channel, javaSocket);
calculateMaxBytesPerGatheringWrite();
}
public DefaultSocketChannelConfig(SocketChannel channel, Socket javaSocket) {
super(channel);
if (javaSocket == null) {
throw new NullPointerException("javaSocket");
}
//保存socket
this.javaSocket = javaSocket;
// Enable TCP_NODELAY by default if possible.
if (PlatformDependent.canEnableTcpNoDelayByDefault()) {
try {
//禁止Nagle算法,目的是爲了讓小的數據包儘量集合成大的數據包發送出去
setTcpNoDelay(true);
} catch (Exception e) {
// Ignore.
}
}
}3.分配線程和註冊Selector
服務端啓動初始化的時候ServerBootstrap#init()，主要做了一些參數的配置。其中對於childGroup,childOptions,childAttrs,childHandler等參數被進行了單獨配置。作爲參數和ServerBootstrapAcceptor一起，被當作一個特殊的handle，封裝到pipeline中。ServerBootstrapAcceptor中的eventLoop爲workGroup。
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
//省略了很多代碼.............
@Override
void init(Channel channel) throws Exception {
//配置AbstractBootstrap.option
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
setChannelOptions(channel, options, logger);
}
//配置AbstractBootstrap.attr
final Map<AttributeKey<?>, Object> attrs = attrs0();
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}
//配置pipeline
ChannelPipeline p = channel.pipeline();
//獲取ServerBootstrapAcceptor配置參數
final EventLoopGroup currentChildGroup = childGroup;
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
}
synchronized (childAttrs) {
currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
}
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) throws Exception {
final ChannelPipeline pipeline = ch.pipeline();
//配置AbstractBootstrap.handler
ChannelHandler handler = config.handler();
if (handler != null) {
pipeline.addLast(handler);
}
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
//配置ServerBootstrapAcceptor,作爲Handle緊跟HeadContext
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
//省略了很多代碼.............
}可見，整個服務端pipeline的結構如下圖所示。bossGroup控制IO事件的檢測與處理，整個bossGroup對應的pipeline只包括頭（HeadContext）尾(TailContext)以及中部的ServerBootstrap.ServerBootstrapAcceptor。
當新連接接入的時候AbstractNioMessageChannel.NioMessageUnsafe#read()方法被調用，最終調用fireChannelRead()，方法來觸發下一個Handler的channelRead方法。而這個Handler正是ServerBootstrapAcceptor
它是ServerBootstrap的內部類，同時繼承自ChannelInboundHandlerAdapter。也是一個ChannelInboundHandler。其中channelRead主要做了以下幾件事。
1.爲客戶端channel的pipeline添加childHandler
2.設置客戶端TCP相關屬性childOptions和自定義屬性childAttrs
3.workGroup選擇NioEventLoop並註冊Selector
1）、爲客戶端channel的pipeline添加childHandler
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
private final EventLoopGroup childGroup;
private final ChannelHandler childHandler;
private final Entry<ChannelOption<?>, Object>[] childOptions;
private final Entry<AttributeKey<?>, Object>[] childAttrs;
private final Runnable enableAutoReadTask;
ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs;
//省略了一些代碼。。。。。
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
//該channel爲客戶端接入時創建的channel
final Channel child = (Channel) msg;
//添加childHandler
child.pipeline().addLast(childHandler);
//設置TCP相關屬性：childOptions
setChannelOptions(child, childOptions, logger);
//設置自定義屬性:childAttrs
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
try {
//選擇NioEventLoop並註冊Selector
childGroup.register(child)
.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}
//省略了一些代碼。。。。。
}客戶端channel的pipeline添加childHandler，在服務端EchoServer創建流程中，childHandler的時候,使用了ChannelInitializer的一個自定義實例。並且覆蓋了其initChannel方法，改方法獲取到pipeline並添加具體的Handler。查看ChannelInitializer具體的添加邏輯,handlerAdded方法。其實在initChannel邏輯中，首先是回調到用戶代碼執行initChannel,用戶代碼執行添加Handler的添加操作，之後將ChannelInitializer自己從pipeline中刪除。
public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {
@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
if (ctx.channel().isRegistered()) {
// This should always be true with our current DefaultChannelPipeline implementation.
// The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering
// surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers
// will be added in the expected order.
//初始化Channel
if (initChannel(ctx)) {
// We are done with init the Channel, removing the initializer now.
removeState(ctx);
}
}
}
private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
if (initMap.add(ctx)) { // Guard against re-entrance.
try {
//回調到用戶代碼
initChannel((C) ctx.channel());
} catch (Throwable cause) {
// Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
// We do so to prevent multiple calls to initChannel(...).
exceptionCaught(ctx, cause);
} finally {
ChannelPipeline pipeline = ctx.pipeline();
if (pipeline.context(this) != null) {
//刪除本身
pipeline.remove(this);
}
}
return true;
}
return false;
}
}
2）、設置客戶端TCP相關屬性childOptions和自定義屬性childAttrs
這點在ServerBootstrapAcceptor#init()方法中已經體現
3）、workGroup選擇NioEventLoop並註冊Selector
這要從AbstractBootstrap#initAndRegister()方法開始，然後跟蹤源碼會來到AbstractUnsafe#register()方法
protected abstract class AbstractUnsafe implements Unsafe {
//省略了一些代碼。。。。。
@Override
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
if (eventLoop == null) {
throw new NullPointerException("eventLoop");
}
if (isRegistered()) {
promise.setFailure(new IllegalStateException("registered to an event loop already"));
return;
}
if (!isCompatible(eventLoop)) {
promise.setFailure(
new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
return;
}
AbstractChannel.this.eventLoop = eventLoop;
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);
}
});
} catch (Throwable t) {
logger.warn(
"Force-closing a channel whose registration task was not accepted by an event loop: {}",
AbstractChannel.this, t);
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}
}
//省略了一些代碼。。。。。
}最後調用AbstractNioUnsafe#doRegister()方法通過jdk的javaChannel().register完成註冊功能。
protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {
//省略了一些代碼。。。。。
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}
//省略了一些代碼。。。。。
}4.向Selector註冊讀事件
a）、入口：ServerBootstrap.ServerBootstrapAcceptor#channelRead()#childGroup.register();
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler);
setChannelOptions(child, childOptions, logger);
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
try {
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}b）、實際上調用了AbstractChannel.AbstractUnsafe#register0()，觸發了通道激活事件;
//觸發通道激活事件，調用HeadContent的
pipeline.fireChannelActive();c）、pipeline的頭部開始，即DefaultChannelPipeline.HeadContext#channelActive()從而觸發了readIfIsAutoRead();
@Override
public void channelActive(ChannelHandlerContext ctx) {
ctx.fireChannelActive();
readIfIsAutoRead();
}d）、讀事件將從尾部的TailContent#read()被觸發，從而依次執行ctx.read()，從尾部開始，每個outboundHandler的read()事件都被觸發。直到頭部。
@Override
public final ChannelPipeline read() {
tail.read();
return this;
}
@Override
public ChannelHandlerContext read() {
//獲取最近的outboundhandler
final AbstractChannelHandlerContext next = findContextOutbound();
EventExecutor executor = next.executor();
//並依次執行其read方法
if (executor.inEventLoop()) {
next.invokeRead();
} else {
Tasks tasks = next.invokeTasks;
if (tasks == null) {
next.invokeTasks = tasks = new Tasks(next);
}
executor.execute(tasks.invokeReadTask);
}
return this;
}
e）、進入頭部HeadContext#read()，並且最終更改了selectionKey,向selector註冊了讀事件
HeadContext#read()
@Override
public void read(ChannelHandlerContext ctx) {
unsafe.beginRead();
}AbstractChannel#beginRead()
@Override
public final void beginRead() {
assertEventLoop();
if (!isActive()) {
return;
}
try {
doBeginRead();
} catch (final Exception e) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireExceptionCaught(e);
}
});
close(voidPromise());
}
}AbstractNioMessageChannel#doBeginRead
@Override
protected void doBeginRead() throws Exception {
if (inputShutdown) {
return;
}
super.doBeginRead();
}AbstractNioChannel#doBeginRead()
@Override
protected void doBeginRead() throws Exception {
// Channel.read() or ChannelHandlerContext.read() was called
final SelectionKey selectionKey = this.selectionKey;
if (!selectionKey.isValid()) {
return;
}
readPending = true;
final int interestOps = selectionKey.interestOps();
if ((interestOps & readInterestOp) == 0) {
selectionKey.interestOps(interestOps | readInterestOp);
}
}參考文章：
Jorgezhong
總結
Netty如何接入新連接基本流程如上所述，如果有誤，還望各位指正。建議先從前兩篇看起比較好理解點。
最後
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